This invention relates to purification of liquid water. In one aspect, this invention is directed to removal of ammoniacal nitrogen from aqueous streams in fish hatchery and fish culture operations.
The world's need for clean water and clean air has led to extensive investigation of the use of ion-exchange materials to remove ammoniacal nitrogen from many fluids. Zeolite ion-exchange has been widely studied, and installations are in place employing zeolites to remove deleterious ammonia from water streams. However, to simply transfer the ammonia from a fluid stream to a solid, such as a zeolite, cannot be economical unless the zeolite can be regenerated and reused. Regeneration processes that have been proposed heretofore involve release of the ammoniacal sorbate from the zeolite ion-exchanger by washing with aqueous solutions containing sodium cations with, as needed, some pH control. Such processes, at best, simply transfer the ammoniacal nitrogen from one stream to another and engender the further problems of disposal. The method of avoiding the disposal problem has generally relied on conducting the transfer of the ammoniacal nitrogen to the ion-exchanger in a manner so as to achieve highest possible concentration loading in the exchange material so that the known regeneration techniques would be feasible and effective. These considerations have led to the use of fixed bed type ion-exchange and regeneration processes.
It has now been found that one can effectively regenerate certain ion-exchange materials effectively in a thermo-chemical regeneration process at elevated temperature and in a continuous manner whereby the ammoniacal nitrogen is destroyed and the ion-exchange material is essentially ready for reuse in the sorption stage. In fact, in many applications the ammoniacal nitrogen can be used at least in part as fuel to effect regeneration.
A very important function of any water quality maintaining system in aquaculture is the removal of ammonia, the major nitrogenous excretion product of fish. Continuous exposure of fish to unionized ammonia at concentration levels of 0.5 parts per million (ppm) ammonia nitrogen with oxygen levels below 5 ppm results in a reduction of fish growth rate, damage to gill tissue (hyperplasia), and an increased susceptibility to bacterial gill disease and other tissue lesions.
The use of zeolites as ammonium ion exchangers for recirculating fish hatchery waters and in fish culture systems is known. Such use has permitted more fish to be raised in the same volume of water as in the past. As a result, biomass production has increased and the overall energy requirements of the system have been decreased. The foregoing uses are described in Pond & Mumpton, Eds., Zeo-Agriculture, Westview Press, Boulder, Colo., U.S.A. (1984), pp. 221-252. Also, U.S. Pat. No. 3,723,308 to Breck and U.S. Pat. No. 4,344,851 to Sherman et al. disclose zeolitic ion exchange materials suitable for aquaculture applications.
However, all such disclosed methods teach periodic, as distinguished from continuous, regeneration of the zeolitic material. Additionally, regeneration of the zeolitic material is effected with an aqueous solution of an alkaline or alkaline earth cation which reconstitutes the zeolitic material not only with respect to sorbed ammonium ions but also with respect to other cations present in the recirculating water. Thus, the normal cation balance of the involved aqueous medium, and thus its "hardness," may be undesirably disturbed.
The foregoing regeneration methods are complicated and expensive to operate on a commercial scale, however. Additionally, such regeneration methods produce a waste effluent that must be disposed of in some manner, contributing a further cost factor to the overall operation.
Moreover, in use the zeolitic ion exchange material also functions as a biofilter medium and accumulates organic debris and bacterial growth thereon, e.g., the slime bacteria Sphaerotilus sp., with attendant fouling and clogging, thereby necessitating frequent, period backwash of the zeolitic ion exchange material beds.
The present invention mitigates, and in some respects obviates, the foregoing shortcomings of heretofore known systems for maintaining the quality of water in aquaculture applications.